1. Field of the Invention
The present invention relates generally to data transmission systems, and more particularly to transmitting data via a modulated signal based upon a signal constellation pattern.
2. Discussion of the Related Art
The increased use of electronic devices (e.g., cellular telephones, modems, etc.) for every day communication has resulted in less available bandwidth and limited frequency allocation for use in these communications. Maximizing use of the allocated bandwidth (i.e., assigned frequencies) is increasingly critical. Specifically, bandwidth modulations that can achieve efficiencies of several bits per second/hertz (bps/Hz) are desirable.
The use of modulated signals to transmit (e.g., broadcast) digital data over analog channels is common. Essentially, digital data is transmitted using a radio-frequency (RF) carrier signal that is modulated (i.e., varied). Various types of modulation schemes are currently used including, for example, amplitude modulation (AM), phase modulation, frequency modulation and pulse modulation, or a combination thereof. Each of these different modulation types is particularly adapted for different applications and has specific inefficiencies that are inherent when modulating signals.
In order to increase the amount of modulated data transmitted, various methods have been developed to pack more data into transmitted carrier waves. For example, quadrature amplitude modulation (QAM) combines two AM signals into a single channel (i.e., modulation of two orthogonal signals in the same carrier signal), which results in doubling the effective bandwidth. QAM provides both amplitude and phase modulation, and is particularly useful for wireless applications, as well as for broadband and modem communication. QAM allows multiple bits to be encoded into a single time interval commonly designated as a symbol period. QAM also allows for both signal amplitude and phase to carry data (i.e., I and Q data carriers). Various forms of QAM are known, including, for example, quadrature phase shift keying (QPSK) and M-ary QAM, which provides multilevel modulation.
With respect specifically to QAM, different transmission schemes exist that are defined by signal constellations having different numbers of code or transmission points per symbol to be transmitted (e.g., 64-QAM and 16-QAM). For example, in 16-QAM, sixteen code or transmission points are available for symbol selection. Essentially, designations such as 16-QAM refer to the size of the signal constellation used to transmit data. The signal constellation provides a plot of all possible QAM signal points with the number of n-bit symbols mapped in a two-axis plane. The constellation diagram shows the possible states for transmission wherein signal constellation diagram shows the possible states for transmission wherein signal amplitude is defined by the distance from the center of the constellation pattern to a point representing a particular state, and the angle created by a straight line connecting the center of the constellation pattern to the point representing the particular state defines the phase angle of the signal.
QAM constellation size (i.e., the number of points in the QAM constellation) is determined as follows: 2 raised to the power of the number of bits per symbol. For example, 16-QAM, which can also be identified as QAM-16, has four bits per symbol (i.e., 24) and 64-QAM or QAM-64 has six bits per symbol (i.e., 26). Thus, using QAM, multiple bits are packed into a single symbol period, such that the value of a symbol consisting of multiple bits is represented by the amplitude and phase states of the carrier wave.
Increasing the efficiency of bandwidth communication is desirable in many applications. One method, for example, is to use non-linear amplifiers which are inherently more power efficient at creating RF energy from direct current (DC) energy. However, achieving a high communication efficiency through a peak-power limited non-linear channel often requires complex controls. The problem encountered is that the use of non-linear channels creates distortions, which makes it difficult (i.e., complex control required) to use known signal constellation patterns, such as M-ary Quadrature Amplitude Modulation (QAM) constellations for transmissions. These known signal constellations are typically arranged in a rectangular pattern.
In particular, the use of non-linear channels causes these known rectangular constellations to rotate and expand in a manner proportional to the distance rectangular. This warping of the constellation results in a greater number of transmission errors because of control problems. Most known methods for compensating for this expansion and rotation are complex, and often inadequate to ensure acceptable transmission reliability levels for specific applications.
What is needed is a transmission scheme requiring less complex control that provides more efficient data communication using modulated signals. In particular, a constellation that retains its basic shape when transmitted over non-linear channels is desirable.